1. Field of the Invention
The invention relates to a rolling bearing unit.
2. Description of Related Art
As a bearing used for, for example, a spindle of a machine tool, there has been known a bearing including a lubrication mechanism of grease sealed type. As a rolling bearing unit including such a bearing of grease sealed type, there has been known a rolling bearing unit configured as described in Japanese Patent Application Publication No. 2006-105345 (JP 2006-105345 A). The rolling bearing unit described in JP 2006-105345 A includes a grease reservoir and a clearance forming piece that defines a clearance which extends from the grease reservoir to a position near a raceway surface of a bearing ring and through which a base oil of grease is guided to the bearing ring under a capillary action.
The inventor of the invention carried out an endurance test with the use of experimental equipment that simulates a rolling bearing unit of grease sealed type. As a result, the inventor found that there is a possibility that a void (crack) will be formed in grease when a thermal history of the grease exceeds a predetermined time. In addition, the inventor found that the narrower the space in which the grease is filled, the higher the frequency of formation of such a void. For example, as illustrated in FIG. 4, if a clearance S2 defined by an inner peripheral face 202 of an outer ring spacer 201, an inner peripheral face 204 of an outer ring 203 and a clearance forming piece 205 is narrow, there is a possibly that a void S will be formed in grease G provided in the clearance S2. If such a void S is formed, the grease G in the grease reservoir 207 and the grease G applied to the outer ring 203 are isolated from each other. This causes a possibility that the supply of a base oil contained in the grease G to the outer ring 203 will be interrupted, and as a result, the supply of the grease G onto a raceway surface (not illustrated) of the outer ring 203 will be insufficient.
In a machine tool required to have high speed rotation performance, it is necessary to continue to supply grease into a bearing with no interruption.